Preparations of carboxylic acids

 Functional group transformations

Primary alcohols and aldehydes are converted to carboxylic acids by oxida-tion. Acid chlorides, acid anhydrides, esters, and amides can be hydrolyzed to their parent carboxylic acids, but only the hydrolysis of esters serves a useful synthetic role if the ester is being used as a protecting group.

C–C bond formation

Aromatic carboxylic acids are obtained by the oxidation of alkyl benzenes.

Alkyl halides can be converted to carboxylic acids where the carbon chain has been extended by one carbon unit. Two methods are possible. The alkyl halide can be converted to a cyanide which is then hydrolyzed. Alterna- tively, the alkyl halide can be converted to a Grignard reagent then treated with carbon dioxide. A range of carboxylic acids can be prepared by alkyl- ating diethyl malonate, then hydrolyzing and decarboxylating the product.

Bond cleavage

Alkenes  can  be  cleaved  across  the  double  bond  by  potassium  perman- ganate. Carboxylic acids are formed if a vinylic proton is present.

 

Functional group transformations

Carboxylic acids can be obtained by the oxidation of primary alcohols or aldehydes, the hydrolysis of nitriles , or the hydrolysis of  esters  which  can  be  used  as  protecting  groups  for  carboxylic acids.  Amides  can  also  be  hydrolyzed  to  carboxylic  acids.  However,  fiercer reaction conditions are required due to the lower reactivity of amides and so amides are less useful as carboxylic acid protecting groups.

 

Although acid chlorides and anhydrides are easily hydrolyzed to carboxylic acids,  the  reaction  serves  no  synthetic  purpose  since  acid  chlorides  and  acid anhydrides are synthesized from carboxylic acids in the first place. They are also too reactive to be used as protecting groups.

C–C bond formation

Aromatic carboxylic acids can be obtained by oxidation of alkyl benzenes. It does not matter how large the alkyl group is, since they are all oxidized to a benzoic acid structure.

There are two methods by which alkyl halides can be converted to a carboxylic acid and in both cases, the carbon chain is extended by one carbon. One method involves substituting the halogen with a cyanide ion , then hydrolysing the cyanide group. This works best with primary alkyl halides.

The other method involves the formation of a Grignard reagent which is then treated with carbon dioxide (Fig. 1b).

The mechanism for the Grignard reaction is similar to the nucleophilic addition of a Grignard reagent to an aldehyde or ketone.

A range of carboxylic acids can be prepared by alkylating diethyl malonate, then hydrolyzing and decarboxylating the product.

Bond cleavage

Alkenes can be cleaved with potassium permanganate to produce carboxylic acids (Fig. 4). A vinylic proton has to be present, that is a proton directly attached to the double bond.